This invention relates in general to a chemical vapor deposition method for depositing thin film chromium compounds. More specifically, the invention deposits a highly spin-polarized thin film of a chromium oxide on a non-magnetic substrate utilizing chromyl chloride as a precursor.
Spin polarized transport effects in materials has become an important and rapidly developing area of basic research and technology. This new field, known as magnetoelectronics, is spurring the development of new devices which cannot be realized with existing semiconductor based electronics. A central component of these devices is ferromagnetic materials which are ideally 100% spin polarized, in which the conduction electrons have only one spin state. Recent point contact experiments have indicated that the spin polarization in chromium dioxide (CrO2) approaches 100%, as disclosed in an article by R. J. Soulen et al., Science 282, 85 (1998). Ultra-thin layers of highly spin-polarized CrO2 have potential applications in giant magnetoresistance (GMR) devices. It is therefore important to develop an efficient and controlled method for preparing CrO2 films.
CrO2 is a ferromagnet with a characteristic Curie temperature of xcx9c395xc2x0 K that crystallizes with the rutile structure (tetragonal, P42/mnm). Chromium forms many oxides including CrO3, Cr2O5, CrO2, and Cr2O3, with Cr2O3 being the most stable. The fabrication of thin films of single-phase CrO2 is often difficult, requiring epitaxial growth on appropriate substrates. Epitaxial growth is the method by which a layer of material is set down upon a crystalline substrate, and the layer of material has a crystallographic orientation which is identical to that of the substrate. CrO2 has been shown to irreversibly reduce to Cr2O3 at temperatures between 250 and 460xc2x0 C. placing clear temperature constraints on the growth method, as disclosed in the following articles: K. P. Kxc3xa4mper et al. Phys. Rev. Lett. 59, 2788 (1987); L. Ranno et al., J. Appl. Phys. 81, 5774 (1997); and K. Kxc3x6hler et al., J. Solid State Chem. 119, 13 (1995). Despite these temperature constraints, there have been some attempts to prepare CrO2 thin films by chemical vapor deposition (CVD), which is a method used in the manufacture of integrated circuits or optical fibers, whereby a thin solid film of one material is deposited on the surface of another by using a radio frequency or other electrical energy source to dissociate a reactive gas. Examples include chemical vapor transport (CVT) of CrO2Cl2 in a sealed tube at 3 atmospheres pressure, photodecomposition of CrO2Cl2, and photodecomposition of Cr(CO)6 as respectively disclosed in the following articles: L. Ben-Dor et al., J. Cryst. Growth 24/25, 175 (1974), C. Arnone et al., Appl. Phys. Lett. 48, 1018 (1986); and F. K. Perkins et al., Thin Solid Films 198, 317 (1991). The films grown by CVT were not tested to verify that the films were single phase CrO2, and thus it does not appear that these films were of acceptable quality. In the photodecomposition experiments, the CrO2 films formed were not of acceptable quality because they either contained undesirable Cr2O3 or were amorphous in structure rather than crystalline.
More recently, thin film growth efforts have involved CVD using CrO3 as a precursor based on the Ishibashi method which is described in an article by S. Ishibashi et al., Mater. Res. Bull. 14, 51 (1979) or a high-pressure bomb which is described in an article by L. Ranno et al., J. Appl. Phys. 81, 5774 (1997). The CrO3 precursor is a solid that sublimes at xcx9c260xc2x0 C. and also partially decomposes.
Conventional liquid precursor handling equipment for CVD typically utilizes a liquid precursor bubbler, automated valves, a pressure controller, mass flow controllers, and precursor flow sensors to deliver precise quantities of vaporized precursor into the reactor. Due to the solid phase CrO3 precursor, the above referenced methods cannot utilize conventional liquid precursor handling equipment for CVD. Instead, in the above referenced processes for depositing a film of CrO3, a two-zone reactor furnace, having two different temperature zones, 250xc2x0 C. and 400xc2x0 C. respectively was utilized. The CrO3 precursor is sublimed within the first temperature zone, and within the second temperature zone the sublimated precursor is completely decomposed onto the substrate. CrO2 thin films have been prepared on a variety of substrates using this two-zone method. However, with the two-zone method it is difficult to precisely control the growth rate, thickness, quality, and chemical composition of the CrO2 thin film. In addition, the two-zone method makes development of a tri-layer process very difficult. The tri-layer process is of potential significance for manufacturing non-volatile permanent memory devices. Due to the absence of conventional CVD bubblers which are controlled via external valves, with the two-zone method sequential switching of the precursor gases to form three layers cannot be accomplished in a controllable manner.
It would therefore be desirable to provide a CVD method for the preparation of high quality epitaxial CrO2 thin films in a conventional tube furnace with conventional CVD precursor handling equipment which utilizes a precursor that is a liquid at room temperature and has a high vapor pressure.
The present invention provides an efficient and controllable method for depositing high quality CrO2 thin films in a conventional tube furnace with conventional CVD precursor handling equipment with CrO2Cl2 as a precursor. The method produces epitaxial CrO2 films which are metallic, smooth, and highly spin-polarized. This method enhances the possibilities of fabricating GMR and/or tunnel junction devices based on CrO2, and thus opens up new opportunities in magnetoelectronics.
In a preferred embodiment, the method includes: selecting a volatile liquid chromium oxide precursor that decomposes in a heated process chamber to provide a chromium oxide layer on a substrate, placing the volatile liquid chromium oxide precursor in a first bubbler, transporting the volatile liquid chromium oxide precursor vapor with a carrier gas into the heated process chamber having the substrate therein, and growing the chromium oxide layer at a controlled growth rate on the substrate in the heated process chamber. Preferably, the volatile liquid chromium oxide precursor is chromyl chloride, the chromium compound is CrO2, and the substrate is TiO2.
In another embodiment, the chromium compound is Cr2O3 and the substrate is Al2O3.
Other advantages and features of the invention will become apparent from the following detailed description of the preferred embodiments and the accompanying drawings.